1. Field of the Invention
The invention relates to an ultrasonic measuring method for measuring the thickness of a coating material, such as the basis weight of electrode paste applied by coating to a metal foil on an electrode production line in a battery production process, for example, during operation of the line, and an ultrasonic measuring system that implements the ultrasonic measuring method.
2. Description of Related Art
The battery production process includes a step of producing electrodes from an electrode sheet formed by applying electrode paste by coating to a metal foil on an electrode production line. Since the quality of the electrodes has a large influence on the performance of batteries as final products, it is important, for quality control, to conduct quality check regarding the basis weight (or coating weight) and coating profile of the electrode paste, after it is applied by coating to the metal foil. In the related art, samples of a given shape are arbitrarily taken out of the electrodes produced on the electrode production line, and quality check is performed on the samples, outside the production line, by measuring differences in weight between samples taken out before coating with the electrode paste, and samples taken out after the coating. As another example, quality check is performed using a measuring system (which will be called “radiation measuring system”) that uses X rays or β rays.
In some cases, the quality check regarding the basis weight and coating profile of the coating material is desired to be conducted evenly or uniformly over a wide range on the electrode production line, with respect to the electrode paste of the electrode produced in the electrode production line, for quality control. Thus, the inventors of the present invention attempted to make a 100% inspection of the basis weight and coating profile of the electrode paste, on the production line, with respect to all of the electrodes produced on the electrode production line, using an ultrasonic measuring system as disclosed in Japanese Patent Application Publication No. 2008-102160 (JP-A-2008-102160), for example. FIG. 22 is an explanatory view showing the ultrasonic measuring system as disclosed in JP-A-2008-102160. As shown in FIG. 22, the ultrasonic measuring system has a pair of ultrasound sending means 81 and ultrasound receiving means 82, which are placed above a measurement object 90, and incident waves sent from the ultrasound sending means 81 are transmitted through the measurement object 90, so that the ultrasound receiving means 82 receives the reflected waves from the measurement object 90. The ultrasound sending means 81 and the ultrasound receiving means 82 are spot-type sensors that permit propagation of focused ultrasonic waves.
In the ultrasonic measuring system of JP-A-2008-102160, a propagation time measuring means 83 measures the propagation velocity of ultrasonic waves propagated through the measurement object 90, based on an incident signal of the ultrasound sending means 81 and a reflection signal received by the ultrasound receiving means 82, and a velocity correcting means 85 corrects the propagation velocity calculated by the propagation time measuring means 83, based on respective temperatures of a liquid phase 91 and a solid phase 92, which are measured by temperature measuring means 84a, 84b. A propagation path length measuring means 86 measures the thickness of the measurement object 90, and a phase-change position of the measurement object 90 as a laminate of the liquid phase 91 and the solid phase 92, based on the propagation velocity of ultrasonic waves obtained by the propagation time measuring means 83, and the correction value of the propagation velocity obtained by the velocity correcting means 85.
However, the above-described technologies of the related art have the following problems. Since the method of measuring differences in weight between samples (metal foil) that have not been coated with the electrode paste and samples (metal foil) coated with the electrode paste cannot be applied to the electrode production line that is in operation, the method is carried out outside the production line, on electrodes as samples taken out of the electrode production line. However, in this method using differences in weight of the electrode paste, the operator cannot correctly determine the basis weight and coating profile of the electrode paste.
If the radiation measuring system is used for conducting the quality check regarding the basis weight and coating profile of the electrode paste, in the electrode production line, the equipment is available at excessively high cost because radiation measuring systems, in general, are very expensive. Also, the radiation measuring system is arranged to focus radiation from a radiation source, and irradiate a measurement portion of an object to be measured, so as to measure the thickness of the measurement portion in the object to be measured; therefore, the thickness can be measured only within an extremely small region of, for example, Φ0.3 (mm), as a measurable range. While samples of electrodes are taken out of the electrode production line, for measurement of the thickness, the radiation measurement system is not able to measure the basis weight of the electrode paste and the coating profile of the electrode paste with high reliability and high measuring accuracy, even with respect to the electrodes sampled from the electrode production line, since the electrode includes a high-density metal foil.
In the ultrasonic measuring system using spot-type ultrasonic sensors, like the one as disclosed in JP-A-2008-102160, the measurable range that can be measured with the ultrasonic sensors is considerably narrow since the range is reduced toward the measurement portion, as in the above-described radiation measuring system. Therefore, the thickness of the electrode paste, or the basis weight of the electrode paste, can be only locally measured. The examination of the coating profile of the electrode paste is to measure the thickness of the electrode paste over a given range in an edge portion of the electrode paste, so as to grasp the shape or profile of the edge portion as a whole. Therefore, the spot-type ultrasonic sensors cannot measure the coating profile of the electrode paste with high reliability and high measuring accuracy. Thus, the ultrasonic measuring systems of the related art are not able to perform wide-range quality check regarding the basis weight and coating profile of the electrode paste, on the electrodes produced on the electrode production line, evenly or uniformly over a wide range on the production line.